This invention pertains to microstructures, particularly a method of rapid prototyping to create polymeric and metallic high aspect ratio microstructures (xe2x80x9cHARMsxe2x80x9d).
In the last few years, research on low-cost, mass production microfabrication techniques for microelectromechanical systems (xe2x80x9cMEMSxe2x80x9d) devices has been very active. The MEMS research community has adopted and modified conventional polymer forming techniques (e.g., hot embossing and injection molding) to massively replicate micron-scale plastic MEMS devices. Many polymers have been investigated as candidate massive replication materials for MEMS, including polycarbonate, polymethyl methacrylate (xe2x80x9cPMMAxe2x80x9d), polyvinyl chloride, polyethylene, and polydimethylsiloxane (xe2x80x9cPDMSxe2x80x9d), but none are well suited for massive manufacturing of MEMS using conventional massive replication techniques. See H. Becker et al., xe2x80x9cPolymer High Aspect Ratio Structures Fabricated With Hot Embossing,xe2x80x9d Digest of Technical Papers, The 10th International Conference on Solid-State Sensors and Actuators, pp. 1432-1435 (1999) and D. C. Duffy et al., xe2x80x9cRapid Prototyping of Microfluidic Switches in Polydimethylsiloxane and Their Actuation by Electro-Osmostic Flow,xe2x80x9d J. Micromechanics and Microengineering, vol. 9, pp. 211-217 (1999).
With the exception of PDMS, casting of most thermoplastic materials generally requires either a modified injection molding machine or a hot embosing machine. PDMS precursors are generally a mixture of dimethylsiloxane and a curing agent, and are available, for example, under the trademark SYLGARD 184(copyright) (Dow Corning, Midland, Mich.). PDMS largely eliminates the need for injection molding and hot embossing machines to replicate microstructures because it can be casted and fully cured at 65xc2x0 C. Currently, PDMS is mainly used by the MEMS community for microstructure replications in micro total analysis system applications (xe2x80x9cxcexcTASxe2x80x9d). See B. H. Jo et al., xe2x80x9cThree-dimensional Micro-channel Fabrication in Polydimethylsiloxane (PDMS) Elastomer,xe2x80x9d IEEE/ASME J. Microelectromechanical Systems, vol. 9, no. 1, pp. 76-81 (2000).
HARMs are preferably used in micron-scale MEMS applications. HARMs provides a number of advantages to MEMS, such as structural rigidity, lower driving voltage in actuator systems, higher sensitivity in sensor applications, and larger magnetic forces in magnetic MEMS. In xcexcTAS, HARMs provide a higher active surface area per unit substrate surface area, a higher packing density of microstructural elements, and a higher throughput in continuous flow systems due to larger cross-sections per unit substrate area.
Commercial manufactures have traditionally used metallic and ceramic micromolds to replicate polymeric microstructures. Metallic micromolds are generally fabricated using a LIGA process, while ceramic micromolds are usually fabricated using a ceramic casting process. (xe2x80x9cLIGAxe2x80x9d is a German acronym for xe2x80x9clithography, electrodeposition, and plastic molding.xe2x80x9d) The LIGA process is a well-known technique that makes it possible to create HARMs having an aspect ratio of approximately 100:1. While LIGA, in many instances, is a preferred method of fabricating HARMs, it requires access to a synchrotron radiation source, which is generally undesirable in mass reproduction due to costs associated with building and maintaining a synchrotron radiation facility.
H. Lorenz et al., xe2x80x9cHigh aspect ratio, ultrathick, negative-tone near-UV photoresist and its applications for MEMS,xe2x80x9d Sensors and Actuators A-Physical, vol. 64 (1), pp. 33-39 (1998) discloses an alternative method for fabricating micromolds using a LIGA-like process that produces lower resolutions and aspect ratios than can be made with a LIGA process.
C. Chung et al., xe2x80x9cHigh aspect silicon trench fabrication by inductively coupled plasma,xe2x80x9d Microsystem Technologies, vol. 6 (3), pp. 106-108 (2000) discloses another alternative process to fabricating micromolds. This process requires the creation of a deep silicon trench using deep reactive ion etching, based on inductively coupled plasma.
Micromolds are used to manufacture inverse images of plastic microstructures using injection molding or hot embossing techniques. However, metallic and ceramic micromold inserts tend to wear out after repeated use. When a mold wears out, a new mold must be made using an x-ray synchrotron source. Thus, in order to replicate plastic microstructures of the same quality (similar tolerances) micromolds should be replaced regularly. For example, nickel micromolds are usually replaced after approximately 100 injection molding sequences have been performed.
U.S. Pat. No. 6,039,897 describes a method for patterning materials onto a substrate surface using an elastomeric mold formed by placing an elastomeric master in conforming contact with the substrate surface. A micro-molding fluid that is a precursor of the material to be patterned is introduced into elastomeric mold reservoirs and then solidified.
U.S. Pat. No. 6,033,202 describes an elastomeric mold for fabricating microstructures comprising a body of elastomeric material having first and second surfaces. The first surface includes at least one recessed microchannel while the second surface includes at least one mold filling member that extends through the mold to the first surface and communicates with the recessed microchannel.
U.S. Pat. No. 5,976,457 describes a method for rapid fabrication of molds or mold components to be used in die cast and injection tools by using a powder injection molding process and a sintering process to form a full or nearly full density metal die or mold component.
U.S. Pat. No. 5,900,160 describes methods of forming a patterned self-assembled monolayer on surfaces and derivative articles. Self-assembled monolyers are typically formed of molecules each having a functional group that selectively attaches to a particular surface.
U.S. Pat. Nos. 5,580,507 and 5,435,959 describe methods for making a mold using a model. A mold carrier is formed having an aperture with a corresponding shape therein. After positioning the model within the aperture, a resilient first material is poured into the space covering the outer peripheral surface of the model, forming a resilient mold insert. The model is removed from the resilient mold insert and a second material is poured into the aperture of the mold carrier with the resilient mold insert disposed along the inner peripheral surface of the aperture, forming a duplicate replicate of the outer peripheral surface of the model. The duplicate is then removed from the resilient mold insert. A mold is formed from the duplicate and is used to make replicas of the model.
An unfilled need exists for a fast and inexpensive microfabrication technique for mass production of micron-scale HARMs MEMS devices.
We have discovered methods for rapid replication of HARMs and MEMS devices by massively reproducing micromold inserts. These methods are highly precise processes that use polymeric microstructure replication techniques and sacrificial layer etching techniques to fabricate high aspect ratio metallic and polymeric micromold inserts. The novel methods are less expensive than prior processes such as LIGA for fabricating multiple metallic molds. The novel methods use one (or more) initial electroplated micromold inserts to massively reproduce high quality, high aspect ratio inserts.
In one embodiment, following fabrication of an initial micromold insert, high quality, high aspect ratio replications are created by repeatedly casting a replication material directly onto the initial micromold insert to create HARM replications. The HARM replications are coated with a sacrificial layer, and are then electroplated to replicate another set of micromold inserts. (The sacrificial layer is used to separate the reproduced mold insert from the reverse-image PDMS mold.) After the electroplating process is completed, the sacrificial layer is etched away to release the replicated micromold inserts.